In his early years, before his dialogical turn, Martin Buber studied the mystical traditions of Europe and Asia and even considered himself a mystic of sorts. The present paper examines Buber's attitude towards mysticism in the course of his intellectual and personal development, particularly before and during his transitional phase to the philosophy of dialogue. The first section details the formative impact intellectual currents around 1900 had on Buber, focussing on the Neue Gemeinschaft of the Hart brothers in Berlin and Gustav Landauer. In a second step, Buber's concept of mysticism is subject to scrutiny. This is essential for understanding why he dissociated so vehemently from mysticism in his dialogical phase. The critique of mysticism in I and Thou is, therefore, the subject matter of the third and last section.